The boron neutron capture therapy (BNCT) is a medical treatment achieved for the purpose of selectively killing cancer cells by “α” rays (helium nucleus) and secondary particle radiation of lithium nucleus (7Li), which are generated through neutron capture nuclear reaction from 10B of a boron isotope (10B) compound having a large neutron capture cross section, which is introduced into the cancer cells in advance by irradiating thermal neutrons having a relatively low energy, such as, equal to 0.5 eV or less, upon the cancer cells. Because the range of the “α” rays and so on is very short, only the cells taking 10B therein are destroyed, and therefore this attracts attention as a medical treatment against the cancer and produces less unfavorable side effects.
In an initial boron neutron capture therapy (BNCT), there are used the neutrons from a nuclear reactor through the deceleration thereof. At present, there are two types, i.e., applying the neutrons which are generated by irradiation of protons accelerated through an accelerator upon solid Be cooled with water- and applying the neutrons which are generated by the irradiation of protons accelerated through an accelerator upon liquid lithium. Herein, explanation will be given on the latter, i.e., the method of applying the neutrons which are generated by irradiating the protons upon the liquid lithium.
The method applying the liquid lithium therein as a target material has advantages that it enables the removal of heat continuously by circulating the lithium and that less fission poison is generated through a nuclear reaction by selecting an irradiating energy equal to no more than 8 MeV. In the present invention, 7Li is changed to 7Be upon irradiation, but 7Be turns back to 7Li again after a half-life period of fifty-three (53) days and can be used continuously. Although it is a radioactive material, 7Be is enclosed within a lithium loop under the condition of being dissolved in the lithium and it turns back to non-radioactive 7Li with an elapse of time. Further, determining the irradiating energy of the protons to be no more than 2.0 MeV brings about an area or region for generating neutrons upon which no deceleration is necessary. Therefore, it is preferable to determine the irradiating energy to this, i.e., equal to no more than 2.0 MeV and also equal to no less than 1.881 MeV threshold value energy, at which the neutrons are generated. This 7Be generation in the nuclear reaction depending on the irradiating energy is one over several tens compared to 2.5 MeV, as a source of generating neutrons for use in the boron neutron capture therapy (BNCT), and it is best to apply the protons of this energy band.
In this boron neutron capture therapy (BNCT) is applied a thermal neutron equal to no more than 0.5 eV, which is generated upon the collision of the protons with a lithium target. In this instance, as an example of the reaction in the vicinity of the threshold value at which no moderator is necessary for the neutron, determining the energy of the proton at 2 MeV and current to 20 mA results in that a large thermal energy of 40 kW is given to the lithium. In spite of inputting this large amount of heat therein, it is better for the lithium not to evaporate and, for the purpose of enabling a continuous operation, there are the following necessities. I.e., that the lithium can always pass through a target portion of the neutron source at a high-speed and with a stable thickness, so as to suppress the lithium from increasing in temperature, and that the lithium as the target is always circulated through a lithium loop, which is built to have equipment for removing the heat from lithium in the lithium loop, i.e., a closed loop of lithium. The lithium of the target portion of the neutron source is formed as thin as possible to not disturb the loci of the neutrons generated in the direction of irradiation of the protons, suppressing the attenuation of the neutrons, and further for forming a stable lithium target stream made from a thin laminar flow having a thickness of 0.5 mm, approximately 0.25 mm or greater than that in the depth thereof, into which the protons irradiated, being so-called a “bragg peak”, are absorbed abruptly, and then onto this is hit by a ray of protons, thereby generating the neutrons therefrom.
In such a proton source as mentioned above, collision of the protons upon the flow of lithium target generates the neutrons to be applied for the purpose of the medical treatment. Also, at that time, the proton itself turns back to hydrogen by taking an electron of lithium in the periphery thereof and a part of this hydrogen is dissolved into the lithium, but much thereof reacts with the lithium to become lithium hydride. If assuming that the current of the irradiating protons is 20 mA, the amount of hydrogen is only 6.53 g if all of the protons turn back to hydrogen and, if irradiating thereon continuously for one year and assuming that there is 25 Kg of lithium, for example, and that the entirety thereof becomes lithium hydride, then the amount of the lithium hydride obtained is 52.24 g. This reaches only 0.21% of the amount of lithium and is in the condition of being dissolved into the lithium in the form of lithium hydride. On the other hand, the neutrons generated in the lithium upon irradiation of the protons are taken into 6Li, i.e., the lithium isotope included in the lithium at 7.4%. Therefore, neutron capture 6Li generate tritium, i.e., hydrogen isotope, through the nuclear reaction. The amount of tritium generated upon continuous irradiation of protons for one year is further small, only 2.44 μg, approximately. Nevertheless, the tritium is relatively long in the half-life period thereof, such as 12.3 years. Therefore, it is difficult to discharge this tritium as it is, but rather necessary to remove the tritium, thereby not discharge it into the atmosphere, but it has to be stored or attenuated, if being discharged, in a concentration equal to or lower than a reference value determined according to a law related therewith. A part of this tritium also dissolves into the lithium, similarly, but reacts with the lithium, thereby bringing about tritiated lithium.
In this manner, because the tritium is very low in the quantity thereof and, judging from the amount of hydrogen reacting with the lithium when there is 25 Kg of the lithium, it is in the condition of being dissolved in the lithium in the form of tritiated lithium. Also, hydrated lithium after reacting with the lithium and also the tritiated lithium, are stable at high temperatures. They do not decompose up to 686° C. and therefore is very small in the amount thereof, which is discharged as a gas in a vacuum discharge system or an argon cover gas system.
However, for the tritium generated, since there are cases of it being discharged from the lithium into the vacuum discharge system or the argon cover gas system, even with a very small possibility thereof, with the boron neutron capture therapy (BNCT) mentioned above, it is necessary to remove the radioactive tritium from the lithium loop and not discharge it into the atmosphere. Also, almost all of the hydrogen and the hydrogen isotope generated in a flow of lithium during the time of irradiation become a lithium compound and is hardly decomposable. However, since very little thereof dissolves under the condition of a hydrogen atom, it has a possibility of being gasified in low pressure conditions, etc., for example, that the gas, generated in a flow portion within a vacuum from the target portion up to a quench surface where the pressure is low among flows of the lithium and/or in a quench tank portion, forms bubbles and thereby rises, or that it generates in a vacuum portion of an inlet portion of a pump. The gas generating causes the generation of cavitations in the inlet portion of the pump, or change the flows before and after a nozzle, or brings the flow of lithium to be unstable. Accordingly, also for stably circulating the flow of lithium, there is a necessity of removing the gas of hydrogen and hydrogen isotope.
Conventionally, as the technology for removing the tritium generated from a nuclear power facility, etc., there are already proposed several ones, as described in the following Patent Documents 1 to 4, for example. However, in the conventional arts, no proposal is made of a means for removing the tritium from the lithium loop for such a neutron source as mentioned above. For this reason, in order to propagate such boron neutron capture therapy (BNCT) as mentioned above, widely, in medical facilities, it is desired to develop a lithium target system equipped with a tritium removal device, for enabling the formation of a stable lithium target flow, without diffusing the tritium into the atmosphere, and further to remove the tritium from among the lithium loop, with safety. Also, it is expected to be put in practical use as a target system for use as the neutron source of an accelerator driving type, other than the boron neutron capture therapy (BNCT).
In general, the tritium (T) cannot exist in the form of a tritium molecule where hydrogen exists but almost always exists under the condition of HT, combining with the hydrogen, and it is very hard to separate or divide those from each other, and the characteristics thereof are almost the same too. Therefore, the tritium must be processed together with the hydrogen when trying to remove the tritium from the hydrogen. Among hydrogen isotopes, general hydrogen is called, “light hydrogen”, distinguishable from deuterium and tritium, however herein, the light hydrogen is called only by “hydrogen”. Also, gasses including the hydrogen and the tritium therein are called, “hydrogen isotope gasses” and, in this device, so as to emphasize or distinguish the tritium, the radioactive material, but always including the light hydrogen therein, it is not called “hydrogen isotope gas”, but “tritium” in the case of the removal of the tritium.